Conventionally, there have been a fixing device including a fixing roller and a pressure roller (see JP 10-74017 A). The fixing roller is heated by induction heating. The fixing roller and the pressure roller respectively heats and pressurizes recording paper so as to fix images on the recording paper.
When small-size recording paper is continuously fed, heat is not removed in non-paper feed areas of the fixing roller and the pressure roller. This causes a problem that the temperature of the non-paper feed areas becomes higher than that of paper feed areas.
As a solution to the problem, a temperature sensor which is provided at an end of the fixing roller detects the state of temperature rise in the non-paper feed areas. Based on values that the temperature sensor detects, a cooling fan is operated, and current flow to coils and conveyance of recording materials are stopped.
In the conventional fixing device, a highest-temperature position of the fixing roller in the axial direction thereof varies depending on the size of paper sheets to be fed Therefore, it is difficult to accurately obtain the state of temperature rise in the non-paper feed areas for all sizes of paper sheets including indeterminately formed paper sheets.
Particularly, in the case of feeding the indeterminately formed paper sheets, the state of temperature rise in the non-paper feed area of the fixing roller may be misidentified, which may cause damage such as breakage to the fixing device.